The light reflected from a retroreflective article generally spreads as it leaves the article, assuming a cone-like pattern centered on the path the light traveled to the reflector. Such a spreading is necessary for practical utility of the retroreflective article. For example, light from the headlamps of an oncoming vehicle, reflected back toward the vehicle by a retroreflective sign, must diverge sufficiently as it leaves the sign to reach the eyes of the driver, who is positioned off-axis from the headlamp beam. In conventional cube-corner retroreflective articles, this cone-like spreading of retroreflected light is obtained through imperfections in the cube-corner retroreflective elements (e.g., non-flatness of the faces, unintended tilting of the faces from their mutually perpendicular positions, etc.) and through diffraction caused because the retroreflected light exits through an aperture defined by the base edges of the three reflecting faces (see Stamm, U.S. Pat. No. 3,712,706).
However, the spreading of light from cube-corner retroreflective articles has important deficiencies: the cone of retroreflected light is often too narrow for many uses that require reflected light to be seen farther off-axis; and the three-sided nature of the cube-corner retroreflective elements gives the retroreflected cone of light an undesirable asymmetric shape. The result is that cube-corner retroreflective articles suffer from a variation in retroreflective brightness when viewed from different presentation angles (a glossary of terms is at the end of the specification). These deficiencies can be sufficiently severe that two persons sitting side-by-side in a vehicle passing a sign covered with cube-corner retroreflective sheeting may have distinctly different perceptions as to the brightness of the sign.
Tanaka, U.S. Pat. No. 3,817,596 increases the divergence or spreading of light rays from a cube-corner retroreflective article by deliberately tilting the faces of the cube-corner retroreflective elements out of perpendicularity or orthogonality. As taught in papers such as P. R. Yoder, "Study of Light Deviation Errors in Triple Mirrors and Tetrahedral Prisms," Journal Of The Optical Society of America, Vol. 48, No. 7, July, 1958; N. E. Rityn, "Optics of Corner Cube Reflectors," Theory and Experiment, Page 198 et seq (UDC 538.318:531.719.24;) and H. D. Eckhardt, "Simple Model of Corner Reflector Phenomena, " Applied Optics, Vol. 10, No. 7, July, 1971, such a tilting of the faces results in light reflected by the cube-corner retroreflective element being divided into as many as six different beams that diverge away from the reference axis of the element and thereby spread the light through a broader range of angles.
Although the spreading of light taught in U.S. Pat. No. 3,817,596 increases the observation angles from which the article may be seen by retroreflection, no effort is made to avoid the basic asymmetry that arises from the three-sided nature of a cube-corner retroreflective element. Further, the spreading reduces retroreflective brightness at commonly experienced smaller observation angles, i.e., the narrow angles near the reference axis, because the light that would ordinarily have been directed to such smaller observation angles is spread through an enlarged region of space. Much of the spread light is wasted, since the article will generally not be viewed from points throughout the enlarged space, and this lost light leaves the retroreflective brightness of the article significantly reduced (see FIG. 6 of U.S. Pat. No. 3,817,596).
Heenan, U.S. Pat. No. 3,833,285, changes the divergence or spreading of light from a cube-corner retroreflective article in a different manner, specifically by incorporating into the article a set of special cube-corner retroreflective elements arranged in a row. In each of these special cube-corner retroreflective elements, two of the faces intersect in a line that is aligned along the length of the row of elements, and the dihedral angle at the intersection of the two faces is enlarged beyond the conventional 90 degrees, e.g., to 90.degree. 30', with the result, as discussed in the publications listed above, that light retroreflected by those elements is split into two beams which diverge along the length of the row. It is contemplated that in different elements within the row the dihedral angle may be enlarged different amounts so as to spread light into an elongated pattern.
An important disadvantage of a retroreflective article as taught in U.S. Pat. No. 3,833,285 is the fact that, in many retroreflective articles, such as traffic control signs, it could be distracting to have a single isolated row of retroreflective elements that distributes light in patterns that are noticeably different from those of other retroreflective elements of the article. For example, instead of seeing a uniformly lit retroreflective sign, an observer would see variations in brightness that could distract from an understanding of the information carried on the sign. Further, a product as described in U.S. Pat. No. 3,833,285 requires the precise manufacture of individual pins that are subsequently bundled together to form the row of elements, and it is difficult to precisely form and group together such distinct pins to obtain retroreflection within desired tolerances.
In summary, the previous efforts at changing the pattern or divergence profile of retroreflected light from cube-corner retroreflective articles have still left such articles with basic deficiencies that limit their utility.